An oxide material having a langasite-type structure, especially a single crystal thereof has the properties of a) having a piezoelectric constant several times as large as that of quartz, b) producing less change in propagation velocity of surface acoustic wave with temperature, and c) having a high electromechanical coupling factor. Focusing attention on these properties, the oxide material is used as a material of a piezoelectric vibrator in a piezoelectric device such as an actuator, a SAW filer, an oscillator, a piezoelectric gyro, and a piezoelectric transformer. Here, when the oxide material is used as a piezoelectric device material, it is preferable for the oxide material to have a higher piezoelectric constant, and therefore, it is preferable to be used as a crystal-oriented single crystal. Moreover, the oxide material is considered the most suitable material for use in high temperature environment because the material does not undergo any phase transition in a temperature region from room temperature to the melting temperature.
For the production of such a langasite-type oxide material, especially for the production of a single crystal thereof, a so-called melt growth method including a Czochralski method using resistance heating, high-frequency induction heating, infrared ray concentrating heating, or the like; a Bridgman method; a micro pulling down method; an EFG method; and a floating-zone method is used. Especially, the micro pulling down method is known as a method by which an excellent single crystal can be obtained at a low cost in comparison with other methods because this method takes short time for crystal growth and near net production is possible, and therefore, there is a demand for application of the method to the production technology of the single crystal of the oxide material. However, when melt growth of the single crystal is conducted, a so-called wettability on a boundary surface between a melt of a raw material of the single crystal and a die portion of a crucible is too high (wetting angle is too small) and the melt exhibits such behavior as going up a side surface of the die portion, and therefore, it is regarded as difficult to achieve stable single crystal growth.
Under the circumstances described above, Patent Literature 1 discloses a method of providing a crucible with a structure that controls behavior of the melt of the raw material, thereby allowing stable crystal growth while controlling a shape such as an outer shape of the single crystal of the oxide material. Moreover, Patent Literature 2 discloses a method of selecting a material of the crucible, thereby suppressing the above-mentioned behavior of the melt going up a side surface of the die portion to conduct single crystal growth.